The present invention relates, in general, to casting devices for making investment molds and, more particularly, to a new and useful cap for vessels used in making investment molds for precision casting of dental prosthesis and a method for making investment molds using the cap.
Investment casting, also known as precision casting, or the lost wax casting technique, is used in dental practice to create a casting from a wax pattern that is a duplicate of the object to be cast. The wax pattern is an acurate replica of the shape of a metal or glass dental restoration such as a gold crown, partial denture framework, or metal coping for a porcelain-fused-to metal crown. The wax pattern is attached to a second piece of wax or plastic known as a sprue.
The sprue is attached to a conical sprue former in a base to hold the wax pattern in place. A casing is then placed around the wax pattern and attached to the base to form a vessel or container into which an investment solution is poured. The investment material then hardens around the wax patterns to form the "investment mold". Removal of the base leaves a conical shape in the investment mold which will direct molten metal into the investment mold. The wax, and, if present, plastic is first removed from the mold space by placing the investment mold into an oven to melt and burn out the wax and plastic.
In many known casting mold devices, a metal or plastic casing is force fit into a groove in the base to form the vessel for holding or receiving the investment solution.
The casing may be a circular ring which fits into a circular base or the casing may be an oblong so-called oval-shaped oval which fits into an oblong or oval-shaped base to form the vessel.
Investment molds or metal rings used for investing are normally filled with an investment material by two known methods. In the first method, commonly known as "investing under vacuum", the entire mold is placed in a vacuum environment as the investment material is poured into the mold. This method utilizes intricate equipment for investing under vacuum which is often subject to breakdown; and therefore, it is very easy to create molds that have bubbles and nodules. A nodule is similar to a bubble in size and configuration. The major difference is that bubbles are perfectly round whereas nodules have sharp or irregular projections generating from the surfaces of the casting.
Bubbles are caused by the entrapment of air in the investment material. Nodules are caused by openings in the surface of the investment material where it contacts the surface of the pattern. When the wax is burned out and the mold is filled with liquid metal, the metal penetrates these slight openings in the investment surface and create nodules of solid cast alloy.
Accordingly, nodules and bubbles must be removed from the surface of the casting. This is a time consuming operation involving machining; and could possibly cause damage to the casting during the machining operation.
The research literature states that mixing investment under vacuum and "Static pouring", the investment into the mold produces the least amount of bubbles when compared with alternate techniques.
The second method of investing is "static investing". Static investing involves mixing the investment material under vacuum, releasing the vacuum and pouring the investment material without vacuum into the mold until the patterns within the mold are covered with the investment material.
When static poured, small bubbles within the investment material collect on the outer surfaces of the pattern. Frequently, air is trapped in the bottom of the wax patterns which has a detrimental effect on the process. The trapped air causes the patterns to be cast with the reproduction of the trapped air becoming solid cast alloy after molten metal has been poured into the void left by the bubble within the pattern.
After the investment has been heated in a furnace to eliminate the wax pattern, molten metal will be poured into the void mold to reproduce the wax pattern in solid metal. The molten metal will fill all voids including all areas where air has been trapped, thus all bubbles or voids on, in, or attached to the pattern will be replicated in solid metal.
For any investment method, it is critical that the intersurface of the pattern fit the patient's tooth. This may not occur if bubbles are encapsulated in the investment material because they will adhere to all of the pattern surfaces that the investment contacts.
FIG. 1 illustrates the most common method of making an investment mold in which the investment material 4 is poured into the mold 10 while in a vertical position. It is well-known that there is a tendency for air to be trapped in the bottom of the copings at 6 since the investment material has a tendency to flow over the top of each coping 7 before the air can escape.
In an effort to prevent this problem, a small sable brush is used to pick up small amounts of the investment material 4 in the mold and place the investment on the side wall of each coping pattern. The mold is then vibrated in order to permit the investment material to flow to the bottom of the pattern in order to displace the air in the bottom of the mold prior to filling the balance of the coping.
However, two major difficulties are encountered when using the brush. First, the bristles of the brush must be wet in order to bring them to a point for facilitating the handling of the investment material. The brush is simply dipped in water and shaken a couple of times in an effort to remove the excess water. The investment material is then picked up on the brush and placed on the wax coping. Accordingly, the water contained within the bristles of the brush mixes with the investment material thereby diluting it. This causes a variance in the expansion of the investment material placed inside of each coping pattern. Thus, a variance in the fit of the casting is created which causes misfits and may sometimes require the fabrication of another casting.
It is rare that another casting would have to be made, however, under expanded castings require excess machining in order to make the casting fit the plaster replica of the patients tooth and subsequently the patient's tooth.
The second difficulty encountered when using a brush is that there is always air contained within the bristles of the brush is deposited in the form of small bubbles inside of the coping at the time of investing. These bubbles create a misfit, if left unattended. Accordingly, the bubbles must be removed from the surface of the casting which proves to be very time consuming.
Furthermore, the average working time for investment material is roughly six minutes. Thus, when employing any of the known investment methods, it is difficult to achieve the filling of multiple molds prior to the investment material setting.